Spectroscopy of Rf257

The isotope (257)Rf was produced in the fusion-evaporation reaction Pb-208(Ti-50,n)(257)Rf. Reaction products were separated and identified by mass. Delayed spectroscopy of (257)Rf and its decay products was performed. A partial decay scheme with configuration assignments is proposed based on alpha hindrance factors. The excitation energy of the 1/2(+)[620] configuration in No-253 is proposed. The energy of this 1/2(+) state in a series of N=151 isotones increases with nuclear charge, reflecting an increase in the N=152 gap. This gap is deduced to grow substantially from 850 to 1400 keV between Z=94 and 102. An isomeric state in (257)Rf, with a half-life of 160(-31)(+42) mu s, was discovered by detecting internal conversion electrons followed by alpha decay. It is interpreted as a three-quasiparticle high-K isomer. A second group of internal conversion electrons, with a half-life of 4.1(-1.3)(+2.4) s, followed by alpha decay, was also observed. These events might originate from the decay of excited states in Lr-257, populated by electron-capture decay of (257)Rf. Fission of (257)Rf was unambiguously detected, with a branching ratio of b(Rf)(SF)=0.02 +/- 0.01.

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